Lower Gastrointestinal Tract Surgery: Vol. 2, Open procedures

By Michael Parker

This atlas, containing a wealth of clear operative images, is designed to enable trainee surgeons to visualise the surgical field for procedures specific to open colorectal surgery, thereby facilitating understanding and learning of surgical techniques and avoidance of intra- and postoperative complications. Step-by-step guidance is provided for a wide range of procedures to the colon, rectum, pelvic floor, anus and anal canal, in each case paying meticulous attention to surgical anatomy. The full range of potential indications for open surgery is considered, including benign and malignant tumours, inflammatory bowel disease, appendicitis, diverticulitis, hemorrhoids, anal fistulas and rectal prolapse among others. Whenever considered necessary, additional line drawings are included to aid comprehension of particular steps in the surgery. Readers seeking to improve their understanding of surgical anatomy and how to perform these operative procedures will find the atlas to be an unparalleled source of information and assistance. A complementary book from the same authors focuses on laparoscopic colorectal surgery.

• Language: English
• Publisher: Springer
• Release date: Feb 23, 2021
• ISBN: 9783030608279

Book preview

Part IColon

© Springer Nature Switzerland AG 2021

W. Hohenberger, M. Parker (eds.)Lower Gastrointestinal Tract SurgerySpringer Surgery Atlas Serieshttps://doi.org/10.1007/978-3-030-60827-9_1

1. Surgical Anatomy and Embryology

Sigmar Stelzner¹  , Werner Hohenberger²   and Thilo Wedel³  

(1)

Department of General and Visceral Surgery, Stadtisches Klinikum Dresden, Dresden, Germany

(2)

Surgical Department, University Hospital Erlangen, Erlangen, Bavaria, Germany

(3)

Institute of Anatomy, Christian-Albrechts University of Kiel, Kiel, Germany

Sigmar Stelzner

Email: [e-mail]

Werner Hohenberger

Email: [e-mail]

Thilo Wedel (Corresponding author)

Email: [e-mail]

Keywords

Colon anatomyEmbryology gutMesocolic fasciaMesofascial interfaceArterial blood supply of the colonGastrocolic trunk

1.1 Introduction

Modern colorectal surgery has given new impetus to the recognition of colon anatomy [1]. The cornerstone of the anatomical concept is the persisting embryological unit of an intestinal tube and its mesentery during postnatal life. The typical adult anatomy is the result of embryological rotations and secondary adhesions that simulate a loss of the mesentery in the region of the ascending and descending colon. However, the delineation of the colon/mesocolon unit from the retroperitoneum, the duodenum and in part the pancreas is represented by a thin fascial layer called the mesocolic fascia in accordance with the terminology of the mesorectum and its mesorectal fascia. The mesocolon, either covered by the mesocolic fascia alone or additionally by the peritoneum, provides the matrix for the blood supply, lymphatic drainage and the autonomic nerves of the bowel wall. There is no direct drainage toward the retroperitoneum as terms such as partial or secondary retroperitoneal imply; therefore this concept should be abandoned.

The colon extends from the ileocaecal valve in the right iliac fossa to the rectosigmoid junction at the level of the sacral promontory. This junction cannot be defined exactly because it is characterised by a gradual coalescence of the taeniae into the continuous longitudinal musculature of the rectum. Owing to the embryological rotations the colon exhibits a frame-like position around the small intestine with two flexures, one below the right lobe of the liver (hepatic flexure) and the other close to the lower pole of the spleen (splenic flexure). The right colon and almost the entire transverse colon derive from the midgut, receiving arterial blood supply from the superior mesenteric artery. The left colon and the remaining left part of the transverse colon belong to the hindgut, with the inferior mesenteric artery as the main visceral blood vessel. Between the two flexures, the transverse colon is covered by the greater omentum, which is adherent to the anterior aspect of the bowel. In this way a direct relationship of the transverse colon (mainly derived from the midgut) is established to foregut derivatives such as the stomach, pancreas and duodenum.

1.2 Embryology

Following the end of the third gestational week, the coelomic cavity develops and is lined by mesodermal tissue. With the folding of the endoderm and the visceral mesoderm, the primitive intestinal tube is moulded and connected by a mesodermal tissue membrane—the dorsal mesentery—with the body wall. The epithelium of the intestines is of endodermal origin, whereas the muscular part of the intestinal tube and its serosal covering derive from the mesoderm. Thus the bowel is enveloped by mesodermal tissue. The outermost part, apart from the peritoneum, can be interpreted as the continuous fascial layer of the mesocolic fascia [2]. This mesodermal layer encloses not only the bowel tube (including the small and large intestine, the duodenum and the stomach) but all derivatives such as the liver, the pancreas, the spleen and the omentum. A similar mesodermal layer lines the primitive abdominal cavity corresponding to the parietal fascia underlying the parietal peritoneum (Fig. 1.1). During the subsequent embryological development, some segments of the mobile primitive intestinal tube fuse with the body wall, causing the disappearance of both the visceral and parietal peritoneal surfaces along the affected areas. However, the mesodermal layers that correspond either to the visceral or the parietal fascia remain with the mesofascial interface in between. This observation reflects the anatomical basis for the feasibility of complete mesocolic mobilisation by releasing the embryological adhesions.

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Figure 1.1

Embryo at the end of the fourth week. The surface ectoderm presents like a somatic tube. The embryonic cavity is lined by the parietal mesoderm (later on the parietal plane), which covers the retroperitoneum and the developing aorta and vena cava. The primitive bowel is connected to the posterior body wall by the dorsal mesentery, which extends from the distal foregut to the end of the hindgut. The visceral mesoderm envelops both the mesentery and the bowel tube to form the mesodermal layer of virtually all intra-abdominal organs. (Modified after Sadler [2])

Further development is characterised by rotation, herniation and reduction of the primitive intestinal tube. Rotation starts in the fifth week anticlockwise with the superior mesenteric artery as the pivot. In the sixth week, the intestines herniate into the umbilical cord and return into the abdominal cavity in the ninth week. When the caecum is located in the right lower abdominal quadrant, the 270-degree rotation is accomplished [3]. At this time the parietal mesodermal layer, which eventually transforms into the parietal fascia, covers all primary retroperitoneal structures such as the aorta, the inferior vena cava and the urogenital organs. Adhesion of the ascending and descending mesocolon and of both flexures to the retroperitoneum, the duodenum and partly to the head of the pancreas are secondary phenomena that start in the midline and stretch out in lateral directions [4].

From around the 12th to the 30th gestational week, major changes in the upper abdomen occur; the development of the stomach, the omentum and the pancreas is the most important in relation to the large intestine. The commonly proposed theory of the left-sided rotation of the stomach and the dorsal mesogastrium has been challenged in favour of a different speed and direction of growth in different parts of the stomach that result in the caudal orientation of the greater curvature and the greater omentum [5, 6]. Within this process, the tail of the pancreas is pushed into the transverse mesocolon on the left side, resulting in a fusion that exceeds the kind of fascial interfaces described above [5]. This corresponds to clinical observations that the posterior mesocolic fascia of the descending colon and the left posterior leaf of the transverse mesocolic fascia are continuous with the posterior mesopancreatic fascia. Following this plane, the dissection of the descending mesocolon in a cranial direction mobilises the pancreatic tail. In order to separate the descending and transverse mesocolon from the lower border to the pancreatic tail, this continuous mesofascial layer must be incised. Anteriorly, the continuity of the anterior mesocolic and mesopancreatic fascia is similar and additionally is covered by the mesothelium of the lesser sac. Again, to separate the mesocolon from the pancreas, this layer must be sharply divided. Between the two mesofascial layers, the connective and adipose tissue matrix contains the inferior mesenteric vein, the lymphatics and the small vessels extending between the transverse colon and the pancreas. In this way, a topographic proximity and close connection of the different parts of the embryonal gut are established between approximately the 20th and the 25th week by a mechanism that is not yet fully understood [5].

For this reason, lymphadenectomy of the transverse colon and the splenic flexure needs to address not only colonic lymph nodes but also lymph nodes of the lower border of the left-sided pancreas.

Modern colorectal cancer surgery attempts to preserve the embryological compartment of the colon/mesocolon unit that is delineated by the mesocolic fascia, providing a natural barrier against cancer cell spread. Thus, the philosophy behind the operation is to follow the embryological planes and reverse the secondary adhesions of the primitive gastrointestinal tube caused by the embryological development.

1.3 Fasciae

A key part of the mesorectum as characterised by Richard Heald is its being embedded along the parietal pelvic fascia with an almost blood vessel–free interface between these two layers, the so-called holy plane [7]. This concept can also be applied to the topographic anatomy of other gastrointestinal segments within the abdominal cavity. The parietal pelvic fascia is not an isolated structure confined to the pelvic cavity but represents the most caudal part of a parietal lining covering the entire abdominal cavity. This parietal fascia separates the bilateral, symmetrical somatic individual [8] from the visceral individual and covers all retroperitoneal structures as a continuous plane. It is interrupted only at the confluence of the hepatic veins, the three large visceral arterial trunks originating from the abdominal aorta, the oesophageal opening of the diaphragm and the outlets of the pelvic floor.

Different parts of the parietal fascia were given different terms or eponyms, which has led to a misconception of its integrity. For instance, the fascial covering of the renal adipose capsule is called the anterior renal fascia, Gerota’s fascia or Toldt’s fascia. In his original contribution, however, Toldt did not describe this parietal fascia but referred to the mesentery of the ascending and descending colon (mesocolon) as membrana mesenterii propria. He recognised its integrity from embryological development to postnatal and adult stages and its fusion with the retroperitoneum as being secondary in nature [4]. Irrespective of the given term, the innermost covering of the retroperitoneum corresponds to a continuous fascial structure, of which the anterior renal fascia is only an integral part. It constitutes the dorsal side of the embryological visceral/somatic interface not only towards the colon and mesocolon but also towards the posterior side of the duodenum and pancreas and the bare area of the liver.

The different colonic segments and accompanying mesocolon can be separated and mobilised from the surrounding structures by following the embryological planes without disruption of the integrity of the mesocolic fascia (Fig. 1.2) [9]. For this embryological plane, the term mesofascial interface was proposed [10]. At the flexures, adhesions can be found that have been named hepatocolic and splenocolic ligaments because of their shape and orientation. Culligan and colleagues [9] were able to show that in a narrower sense, ligaments do not exist. These adhesions derive from condensations either of the greater omentum with its mesofascial envelope (meso-omentum) or from duplications of the parietal fascia or sometimes from a combination of both. Even in these areas, the integrity of the embryological compartments and the mesofascial interface is maintained.

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Figure 1.2

(a) Peritoneal reflection on the left side (line of Toldt). (b) Mesofascial interface on the left side. The left-sided colon and the descending mesocolon have been detached from the parietal abdominal fascia (synonym: anterior renal fascia, Gerota’s fascia). The inferior mesocolic vein (IMV) is clearly visible and covered by the mesocolic fascia. The ovarian vessels and the ureter are covered by the parietal abdominal fascia. Either side of the mesofascial interface exhibits a smooth, shiny surface. Intraoperative situs

1.4 Mesocolon

The mesocolon represents the colonic part of the dorsal mesentery and keeps the bowel tube in place according to the degree of adhesion with the parietal fascia. It consists of two layers of mesocolic fascia on either side with a matrix of connective tissue and visceral adipose tissue of varying size in-between. This matrix houses blood and lymphatic vessels, lymph nodes and nerves supplying the bowel tube [11]. The mesocolic fascia continues as a thin subserosal envelope on the bowel surface. Whereas the areas of the bowel tube and the mesocolon that face the abdominal cavity or the lesser sac are covered by peritoneum, the peritoneum has disappeared in the areas of adhesion to the parietal abdominal wall and the viscera, e.g., the duodenum or the pancreas. Rarely, some peritoneal inclusions can be found when the mesocolic interface is opened up as a sign of incomplete embryological adhesion, especially on the left side. Whereas the mesocolic adipose tissue is usually well developed along the large mesocolic vessels, it can be reduced or even missing in vessel-free areas. This is frequently the case in the ascending and descending mesocolon, which sometimes only consist of connective tissue of the mesocolic fascia and the mesothelial layer of the peritoneum. These areas are also called mesocolic windows.

The mesocolon is contiguous with the mesentery of the small intestine at the ileocaecal junction and continues into the mesorectum at the rectosigmoid junction. The mobile ileal mesentery ends shortly before the ileocolic vessels that mark the beginning of adhesion of the ascending mesocolon. The caecum is mobile in a large proportion of individuals to a varying degree. The transition of the parietal into the visceral peritoneum of the ascending (and likewise the descending) colon is marked by a sometimes sharp tissue condensation that is referred to as the line of Toldt (cf. Fig. 1.2a). This line must be incised for mobilisation of the large bowel to enter into the mesofascial interface between the parietal and the visceral fasciae. This interface continues medially behind the duodenum, the head of the pancreas and the uncinate process and is used in the Kocher manoeuvre. However, a similar interface exists on the anterior side between the mesofascia of the ascending mesocolon and the pancreaticoduodenal unit (Fig. 1.3). This plane can be followed close to the superior mesenteric vein. The matrix of the ascending mesocolon reaches the superior mesenteric vein and artery centrally between the emergence of the superior mesenteric vein at the inferior border of the pancreatic head and the ileocolic vessels. The posterior ascending mesocolic fascia covers and includes the root of the superior mesenteric vein up to the gastrocolic trunk and must be incised when the vein is to be dissected. Since this part of the superior mesenteric vein from the gastrocolic trunk to the ileocolic vein serves as an anatomical landmark for the dissection of the ascending colon in right-sided colectomies, it is called the surgical trunk by some authors [12].

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Figure 1.3

Mesofascial interface on the right side. The right-sided colon, including the hepatic flexure and the ascending mesocolon, have been mobilised and lifted to the left. The mesocolic fascia (mesocolic plane) and the fascial layer over the duodenum and the pancreas (mesoduodenum, mesopancreas) are intact. Note the smooth surface of the mesocolon with the venous vessels underneath the shiny ascending mesocolic fascia. The pancreaticoduodenal vein (PDV) and the superior right colic vein (SRCV) converge to form the gastrocolic trunk (intraoperative situs)

The upper side of the transverse mesocolon is covered by the greater omentum. Particularly on the right side a wide area can be adherent but separable following the mesofascial interface. On the left side, the lesser sac is interposed between the greater omentum and the transverse mesocolon. The relationship of the transverse mesocolon to the lower border of the pancreatic tail is peculiar because of the continuity of the mesofascial layer with the mesopancreas on the posterior and anterior sides and the vascular and lymphatic connections in-between. Separation of the transverse mesocolon from the pancreatic tail comprises an incision of the two mesofascial layers and a dissection of the connective tissue matrix in-between, including the inferior mesenteric vein and sometimes the small arterial vessels (Fig. 1.4). Again, the thickness of the mesofascial matrix varies individually and depends mainly on the amount of embedded visceral adipose tissue, which is most pronounced around the large vessels.

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Figure 1.4

(a) The lesser sac is fully opened after detaching the greater omentum from the transverse colon and lifting it upward. The anterior mesocolic fascia of the transverse mesocolon is fused with the anterior mesopancreas along the pancreatic tail. The continuous mesofascial layer is partly incised from the left. Detachment of the transverse mesocolon has to follow the interrupted line. (b) In the same specimen, the transverse colon and mesocolon are lifted upwards. The posterior fusion of the transverse mesocolic fascia and the mesopancreas is severed along the interrupted line. SMV—superior mesenteric vein. Formalin-fixed specimen

The left-sided mesocolon rests on the parietal fascia covering the adipose renal capsule. The inferior mesenteric artery is surrounded by the autonomic nerve plexus, lymphatic vessels and to a varying degree by connective and adipose tissue. At the level of the inferior mesenteric artery, an adhesion between the base of the mesentery of the small intestine and the medial side of the left mesocolon can be encountered quite frequently (Fig. 1.5). The descending and the sigmoid mesocolons may sometimes even reach the wall of the small intestine. Again, a separation following the embryological planes is possible.

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Figure 1.5

Embryological adhesion between the mesentery and the descending mesocolon from the duodeno-jejunal flexure (white star) to the root of the inferior mesenteric artery. The area of adhesion is larger than usual. If the planes are to be correctly detached during mobilisation of the left colon, the incision must start at the dotted line. Intraoperative situs

The shape of the mesosigmoid is fan-like owing to the variable length of the sigmoid. The main portion is mobile, while the central part is fixed in the area of the left common iliac vessels to a varying degree. Deep peritoneal pockets on the posterior side may develop as a result of superficial secondary peritoneal fusions. The mesosigmoid is continuous with the mesorectum. Distal to the origin of the inferior mesenteric artery from the aorta, the mesenteric interface can be followed to the right side, separating the vascular pedicle of the superior rectal artery from the parietal fascia covering the autonomic superior hypogastric plexus. Seen from the left side, the superior rectal artery can regularly be identified superficially within the mesorectum, covered only by the mesocolic/mesorectal fascia (Fig. 1.6). Following the pedicle caudally provides access to the embryological interface between the mesorectal fascia and the inner lamella of the parietal pelvic fascia [13].

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Figure 1.6

Pedicle of the superior rectal artery (SRA) within the sigmoid mesocolon. The mesocolic interface has been detached, revealing the superior hypogastric plexus and the common iliac artery, which are covered by the parietal fascia. IMV Inferior mesenteric vein, LCIA Left common iliac artery. Intraoperative situs

1.5 Arterial Blood Supply

The arterial blood supply of the colon is provided by branches of the superior and inferior mesenteric arteries. The ileocolic artery derives distally from the right side of the superior mesenteric artery as a constant branch to the ileocaecal junction. The right colic artery, the middle colic artery, the left colic artery and the sigmoid arteries show a high variability.

Many investigations have been performed to clarify the varying courses of these arteries and conflicting opinions exist, in particular regarding the right and the middle colic arteries [14–16]. Important studies about the variations of these arteries in origin and number are highlighted in Table 1.1. Recently, a comprehensive review and meta-analysis was published, which is used as a reference for Fig. 1.7 [20]. The right colic artery originates directly from the superior mesenteric artery in only 15–40% of cases, whereas it is absent in about 40%; in half of the remaining cases it derives from the ileocolic artery and in the other half, from the middle colic artery. Two (or rarely even three) middle colic arteries can be found in approximately 10%. Variations in the origin of the middle colic artery from the dorsal pancreatic artery and the splenic artery have been observed [19]. Most commonly, the middle colic artery has a short trunk and forks into two branches that may run in a parallel fashion towards the transverse colon before turning to the right and left sides (Fig. 1.8).

Table 1.1

Variations and number of the right and middle colic artery

ICA Ileocolic artery, MCA Middle colic artery, SMA Superior mesenteric artery

aThe authors introduced the term right colic–middle colic trunk as a common origin for the right and middle colic artery

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Figure 1.7

Schematic description of the course and variability of colic arteries. The percentages shown for the right-sided colic arteries and the middle colic artery were taken from Negoi et al. [20]

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Figure 1.8

Anatomy of the proximal superior mesenteric artery (SMA) and the middle colic artery (MCA). The mesocolon has been detached from the lower border of the pancreas and duodenum to expose the proximal superior mesenteric artery (SMA) and its branches. The MCA gives off the right colic artery (RCA) and forks into two branches. IPDA Inferior pancreaticoduodenal artery, JA Jejunal arteries, RGEA Right gastroepiploic artery. Formalin-fixed specimen

The left colic artery can be found constantly as the first branch of the inferior mesenteric artery; sometimes it shares a trunk with the first sigmoid artery [15, 18, 19]. However, the course of the artery may vary, as Goligher [21] describes an ascending left colic artery that runs in its first part close to the inferior mesenteric vein and turns toward the splenic flexure in its second part. In this concept, the next branch is considered as the proper left colic artery or the colosigmoid artery [16, 21]. Concerning the sigmoid arteries, a variable number of one to five has been described in inferior mesentericography, with the vessels originating either directly from the inferior mesenteric artery or together with the left colic artery [19].

The anastomotic connections between the colic arteries and the bowel tube are established by the marginal artery (often called the marginal artery of Drummond) that runs for a short distance along and parallel to the colon. It connects the territories of the superior and the inferior mesenteric arteries in the region of the left-sided transverse colon and the splenic flexure. Several authors have investigated this backbone of colonic blood circulation (Figs. 1.7 and 1.9) [15, 22, 23]. The anastomosis between the middle and left colic artery at the left flexure is commonly summarised under the term Arc of Riolan. More precisely, a peripheral and a central arterial arcade can be discerned, the first being identical with the marginal artery of Drummond [24]. Sometimes only one arcade runs along the colon at the splenic flexure. About 10% of all individuals have an accessory anastomosis that directly connects the superior mesenteric artery and the inferior mesenteric artery, as a shortcut in the colonic root (intermesenteric arch) [17]. It originates from the left side of the superior mesenteric artery or the middle colic artery and joins the inferior mesenteric artery or the left ascending colic artery (Fig. 1.10). Very rarely (3–5%), the middle colic artery is completely missing; this might cause problems in the blood circulation of the proximal left colon after ligation of the inferior mesenteric artery in oncological resections [15, 18]. Likewise, the marginal artery as a rule does not connect the territories of the distal sigmoid arteries and the superior rectal artery (Sudeck’s point). This provides the rationale to use only the rectum for anastomosis once the inferior mesenteric artery has been tied for oncological or other surgical reasons [18, 23].

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Figure 1.9

Intraoperative photo (a) and pictogram (b) illustrating the arterial blood supply of the left-sided colon. The marginal artery is served by the left colic artery, the ascending left colic artery and the middle colic artery. The middle colic artery and the ascending left colic artery form the anastomosis in the region of the splenic flexure. This case exhibits both the peripheral marginal artery (Drummond) and the more central anastomosis between the middle and the left ascending colic arteries (Riolan). IMA Inferior mesenteric artery, SRA Superior rectal artery

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Figure 1.10

Intermesenteric arch (arteria colica sinistra accessoria) between the territories of the superior mesenteric artery and the inferior mesenteric artery crossing immediately along the duodeno-jejunal flexure. The transverse colon is lifted upward by the retractor and covered by the greater omentum. IMV Inferior mesenteric vein. Intraoperative situs

The bowel wall itself is supplied by small terminal arteries that usually originate directly from the marginal artery in a perpendicular fashion. These terminal arteries exhibit short and long branches. Most of the short branches come off the long branches and pass straight through the circular muscle to the submucosa on the mesocolic side of the bowel. The long branches run beneath the serosa toward the free taeniae on either side to penetrate the circular muscle just adjacent to the taeniae (see Fig. 1.9). They cross the appendices epiploicae at the base and are prone to damage if the appendices are tied close to the bowel wall. There are anastomoses between the long and the short branches and some between the two long branches at the submucosal level but only a few in the longitudinal direction [15, 18, 25].

1.6 Venous Drainage

The anatomy of the veins draining the colon is even more variable than it is for the colic arteries. The superior mesenteric vein unites with the splenic vein to form the portal vein. Variants exist concerning the inferior mesenteric vein, which as a rule joins the splenic vein behind the pancreatic body. However, direct drainage into the superior mesenteric vein or a tripod of the three is also observed [26]. Sometimes the inferior mesenteric vein drains into a trunk that is formed by large superior jejunal veins. Rarely, the superior mesenteric vein consists of two main trunks [12].

Surgically the most relevant variants are represented by the relation of the right colic and middle colic veins to the gastroepiploic (synonymous with gastro-omental) vein in the region of the hepatic flexure and right-sided transverse colon. The reason for these variants is the embryologically close relationship of the foregut components, such as the greater omentum, duodenum and pancreas with segments of the distal midgut, namely the hepatic flexure and the transverse colon. This close spatial relationship often entails the formation of a joint trunk for the gastroepiploic vein and the colic veins. This gastrocolic trunk was first described by Henle in 1868 and drains into the superior mesenteric vein just below the neck of the pancreas (Figs. 1.11 and 1.12; see also Fig. 1.4a) [27]. In different frequencies this trunk is supplemented by the superior right colic vein, the pancreaticoduodenal veins and rarely by the middle colic vein. This joint trunk represents a topographic fusion of two embryologically different segments: the foregut and midgut.

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Figure 1.11

Superior mesenteric vein (SMV). The mesocolon has been detached from the lower border of the pancreas and duodenum to expose the tributaries of the gastrocolic trunk (white star). In this specimen, the middle colic vein (MCV) drains directly into the SMV. PDV Pancreaticoduodenal veins, RCV Right colic vein, RGEV Right gastroepiploic vein, SRCV Superior right colic vein. Formalin-fixed specimen

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Figure 1.12

Proximal superior mesenteric artery (SMA) and vein (SMV) with the transverse mesocolon flipped upward. The middle colic artery (MCA) divides into two main branches, which run along the transverse colon on either side. The gastrocolic trunk (white star) is formed by the tripod of the right gastroepiploic vein (RGEV), the superior right colic vein (SRCV) and a pancreaticoduodenal vein (PDV) and receives the right colic vein only shortly before it drains into the SMV. The middle colic vein (MCV) drains directly into the SMV, forming a trunk with two jejunal veins (JV). Formalin-fixed specimen

The gastrocolic trunk is an important anatomical landmark in right-sided complete mesocolic excision (CME). To get access to the superior mesenteric vein for central lymph node dissection and to avoid sudden unexpected bleeding, one must be aware of the possible variants. Dissection follows the right colic vein to its junction with the gastroepiploic vein, where it has to be centrally divided [1]. Any further colic vein has to be taken at the same level. Care must be given to all the delicate tributaries to the gastrocolic trunk, which is therefore often referred to as the bleeding point.

The left-sided colic veins rather consistently drain into the inferior mesenteric vein, whose variants concerning the central course have already been outlined. Rarely, the inferior mesenteric vein is represented by two or even three trunks [26].

1.7 Lymphatic System

In their landmark paper, Jamieson and Dobson [28] defined the architecture of the lymphatic system of the colon. They described epicolic, paracolic, intermediate and main lymph nodes and the routes following the colic arteries. Epicolic lymph nodes have only local relevance. As a rule, the colon drains first into the paracolic lymph nodes, from where drainage continues to the intermediate lymph nodes along the colic arteries. These nodes eventually drain into the main lymph nodes that surround the stem of the vessels at their origin [28]. Lymph flow in the network of lymphatic